Fluid flow history and paragenesis along a syn-rift basin bounding fault: the Helmsdale Fault (NE Scotland)

Within syn-rift basinal settings, the juxtaposition of rift-related clastic deposits in the hanging wall of basin-bounding normal faults against a footwall of crystalline basement is a recurrent structural setting where plays for hydrocarbon exploration or carbon storage can be found. Here, fault-controlled fluid flow can significantly influence and change the petrophysical properties of the fault zone and host rocks over time by means of mineralization and cementation, ultimately controlling fluid pathways. Investigating the timing and extent of fluid flow along major faults permits us to better understand the host rock properties and if these can potentially be favourable for subsurface extraction and storage.

Here, we present a detailed investigation of the timing and paragenesis of fluid flow along the well exposed Helmsdale Fault in NE Scotland. The Helmsdale Fault is a major tectonic feature that bounds the western side of the Inner Moray Firth Basin, which developed during rifting in the Late Jurassic. The hanging wall consists of the Late Jurassic (Kimmeridgian-Tithonian) Helmsdale Boulder Beds that are made of alternating debris flow to fault scarp deposits, whereas the footwall is composed of the Helmsdale Granite (Silurian-Devonian). There is ample evidence of paleo-fluid flow along the Helmsdale Fault in the form of calcite cementation and widespread calcite veining in both the hanging wall and in the footwall, locally making up to 5 m thick fault cores of stacked crack-seal veins. U-Pb calcite dating of fossils, veins and cements shows an initial fluid flow event that quickly follows diagenesis in the hanging wall and spans from 147 to 113 Ma, followed by a later reactivation of the fault system between 86-60 Ma. The spatial distribution of the dated calcite veins shows a clear localization over time of fluid flow along the main faults within the footwall.

Carbonate stable isotope analysis, combined with the salinity of the fluid inclusions in the calcite veins, has revealed a marine fluid composition of the calcite vein network over time, irrespective of the structural domain within the fault zone. Furthermore, clumped isotope thermometry shows a gradual temperature increase towards the footwall (35 to 65 °C), but fluid inclusion microthermometry on secondary fluid inclusions also reveals that these fluids could originally have been much hotter (up to c. 80 °C). The variability in the data suggests that two fluid pathways were active at different moments in time, with one being locally sourced in the hanging wall sediments, and the second percolating upwards along the main faults within the Helmsdale Granite. Occurrence of calcite veins derived from meteoric fluids is documented in the youngest dated vein network (60 Ma) and likely related to the later stages of regional uplift.

Our results suggest that the evolution over time of the petrophysical properties of the hanging wall with progressive mineralization and cementation exert a critical control on future fluid pathways as well as localization and style of subsequent fault deformation.